Laminated glass comprising at least two glass sheets and a plasticized poly(vinyl butyral) interlayer film sandwiched therebetween has fundamental characteristics required of laminated glass. For example, it has good transparency, weather resistance, bond strength, and penetration resistance. It hardly allows its fragments to scatter. Thus, it has so far been widely used as the windshields of automobiles or buildings, for instance.
While laminated glass of this kind is excellent in fundamental characteristics such as mentioned above and in safety, it is poor in moisture resistance. Thus, when the above-mentioned laminated glass is used in a high humidity environment, a problem may arise; namely, the interlayer film in the peripheral region of the laminate may whiten, since the peripheral edges of laminated glass are in direct contact with the environmental air.
This phenomenon of blushing is associated with the additive used for adjusting the bond strength between the interlayer film and the glass, as mentioned below.
In order that the laminated glass will sufficiently discharge the functions mentioned above, it is necessary to adjust the bond strength between interlayer film and glass so that it may fall within an adequate range. Thus, if the bond strength between interlayer film and glass is too weak, glass fragments formed upon breakage due to an external shock may peel off from the interlayer film and scatter to increase the risk for injuring the human bodies and other objects. If, conversely, the bond strength between interlayer film and glass is excessively high, the glass and interlayer film tend to break simultaneously upon receiving a shock load whereupon glass fragments accompanying fragments of the interlayer film will scatter, thus increasing the risks for injuring the human bodies and other objects.
On the contrary, when the bond strength between interlayer film and glass is within an adequate range, breakage of glass occurs over a wide area and results in concurrent partial interfacial peeling of the interlayer film and glass from each other and elongation of the interlayer film, and these phenomena are effective in increasing the resistance against shock and penetration.
Thus, in order to insure that, taking a traffic accident involving an automobile as an example, the shock to the driver and/or passenger may be absorbed, the risk for their being hauled through the broken windshield may be prevented or, in the case of an accident related to a building, the penetration of flying objects against the window pane or scattering of broken glass fragments may be prevented, the bond strength between interlayer film and glass must be judiciously controlled within said suitable range.
In view of the foregoing, various bond strength control agents for the interlayer film have so far been investigated in order to adjust the bond strength between interlayer film and glass to a level within an adequate range.
Thus, for example, Japanese Kokoku Publication Sho-46-4270 proposes an interlayer film for laminated glass which comprises a poly(vinyl acetal) resin composition containing 0.2 to 0.8% by weight of water and a specific amount of a specific metal alkylcarboxylate as a bond strength control agent. The bond strength between the interlayer film and glass according to the above proposal is adjusted to an adequate range by varying the proportions of the metal alkylcarboxylate distributed in the superficial layer of the interlayer film and in the inside layer of the interlayer film or varying the water content of the interlayer film.
The metal alkylcarboxylate-containing interlayer film such as proposed in the above publication, however, is low in moisture resistance, and the laminated glass manufactured by using said interlayer film has a problem in that when allowed to stand in a high-humidity atmosphere, it tends to undergo severe blushing due to moisture absorption by the interlayer film as the metal alkylcarboxylate content increases since the interlayer film is in direct contact with air in the peripheral region of the laminated glass. The phenomenon of blushing of the interlayer film may be prevented by decreasing the amount of the metal alkylcarboxylate as far as possible or avoiding the use thereof but, in that case, there occurs a problem crucial for the laminated glass that the bond strength between interlayer film and glass exceeds the proper range and is ready to allow simultaneous breakage or penetration of the glass and interlayer film upon receiving to an external shock load or the like.
In Japanese Kokoku Publication Sho-44-32185, there is proposed an interlayer film for laminated glass which comprises a molded poly(vinyl acetal) resin having a water content of 0.1 to 0.8% and containing 0.01 to 3 parts by weight, per 100 parts by weight of the resin, of at least one organic acid selected from among monocarboxylic acids containing 6 to 22 carbon atoms, dicarboxylic acids containing 4 to 12 carbon atoms, aliphatic monoaminomonocarboxylic acids containing 2 to 6 carbon atoms, aliphatic monoaminodicarboxylic acids containing 4 or 5 carbon atoms, citric acid, and mixtures thereof.
However, this interlayer film has the drawback that the addition of such a carboxylic acid causes the bond strength to change with the lapse of time. Moreover, another problem may arise; the acid may adversely affect the heat resistance and weather resistance of the interlayer film.
Japanese Kokoku Publication Sho-48-5772 discloses a laminate glass comprising at least two glass sheets glued together by means of a plasticized poly(vinyl acetal) resin composition, said plasticized poly(vinyl acetal) resin composition contains the sodium metal salt of an aliphatic carboxylic acid containing 10 to 22 carbon atoms.
Furthermore, in Japanese Kokoku Publication Sho-53-18207, the use is proposed of an alkali metal or alkaline earth metal salt of a monocarboxylic or dicarboxylic acid as a bond strength control agent in the plasticized poly(vinyl acetal) resin interlayer film.
In either of the above two proposals, a metal salt of a carboxylic acid containing a relatively large number of carbon atoms is used as the bond strength control agent, since such salt is readily soluble in the plasticizer contained in the interlayer film.
However, when a metal salt of a carboxylic acid containing a large number of carbon atoms is used as the bond strength control agent, there occurs a problem that the bond strength between interlayer film and glass changes with the lapse of time. Thus, even when the bond strength is adequate initially, the bond strength will gradually decrease with the lapse of time and the glass will readily undergo peeling when it receives a shock. For preventing this decrease in bond strength, it is necessary to mature the interlayer film by storing the same in an atmosphere of 40 to 50° C. for 1 to 2 months, for instance. However, since the interlayer film has tackiness and a tendency toward self-adhesion, it is as a matter of fact difficult to store the interlayer film in such an atmosphere as mentioned above for a long period of time. Even if the maturing is performed, the decrease in bond strength with the lapse of time can be retarded but cannot be made nil, and the problem mentioned above still remains.
Japanese Kokai Publication Sho-60-210551 discloses a laminated glass comprising at least two glass sheets glued together by means of an interlayer film composed of a plasticized poly(vinyl acetal) resin containing, or carrying as adhered thereto, 0.02 to 0.40 part by weight of the potassium salt of a monocarboxylic acid containing 1 to 6 carbon atoms and 0.01 to 0.26 part by weight of a modified silicone oil per 100 parts by weight of said resin. Certain metal salts, however, may cause blushing of the laminated glass due to their coagulation in the form of particles within the interlayer film. Therefore, from the viewpoint of long-term prevention of blushing resulting from moisture absorption, said laminated glass cannot be said to be a perfect one.
In Japanese Kokoku Publication Hei-02-41547, there is proposed a poly(vinyl butyral) sheet in which an alkali or alkaline earth metal salt of formic acid is used as the bond strength control agent. Furthermore, in Japanese Kohyo Publication Hei-06-502594, an interlayer film containing potassium acetate added as a bond strength control agent is used in the examples of its specification.
In the three proposals mentioned above, a metal salt of a carboxylic acid containing a relatively small number of carbon atoms is used to overcome the problems mentioned above in relation to the use of a metal salt of a carboxylic acid containing a large number of carbon atoms.
When a metal salt of a carboxylic acid containing a small number of carbon atoms is used as the bond strength control agent, the problem of the decrease in bond strength between interlayer film and glass with lapse of time can indeed be solved but the moisture resistance of the interlayer film becomes insufficient and, as a result, another problem arises, namely the peripheral (edge) region of the laminated glass tends to undergo blushing due to absorption of moisture.
More specifically, the interlayer film is generally capable of absorbing moisture under ordinary atmospheric (humidity) conditions and, therefore, in using it in the manufacture of a laminated glass, it is common practice to submit the interlayer film to the lamination process after adjusting its water content to not more than about 0.5% by weight in an atmosphere of 25% RH, for instance. Since, however, the peripheral region of laminated glass are generally exposed, the interlayer film absorbs moisture in a high-humidity environment, whereby the water content increases to about 2 to 3% by weight. On that occasion, water gathers around minute crystals of said metal salt of a carboxylic acid containing a small number of carbon atoms, such as potassium acetate, magnesium acetate or potassium formate, as occurring in the interlayer film, to cause blushing. If the addition amount of the carboxylic acid containing a small number of carbon atoms or a salt thereof is decreased to prevent blushing, the bond strength between interlayer film and glass will deviate from the proper range, hence the shock absorbing potential, penetration resistance and other properties of the laminated glass will become insufficient.
In Japanese Kokai Publication Hei-05-186250, an attempt is made to improve the carboxylic acid salt-containing interlayer film in respect of blushing by using an interlayer film for laminated glass which is composed of a resin composition comprising a poly(vinyl acetal) resin, a plasticizer, an alkali or alkaline earth metal salt of a mono- or dicarboxylic acid containing not more than 12 carbon atoms and an organic acid.
Furthermore, in Japanese Kokai Publication Hei-07-41340, an interlayer film for laminated glass is proposed which is formed from a resin composition comprising a poly(vinyl acetal) resin, a plasticizer, a carboxylic acid metal salt and a straight-chain fatty acid.
The laminated glass including the interlayer film for laminated glasses according to the above proposals show reduced degrees of blushing in the peripheral region in moisture resistance testing but the extent of reduction in blushing is yet unsatisfactory. Moreover, if the content of the straight-chain fatty acid is increased for further reducing the degree of blushing, foaming and/or discoloration may possibly occur when the laminated glass is exposed to a relatively high temperature.
While the interlayer films proposed in the above-cited publications are results of attempts to solve the blushing problem by improving the bond strength control agent, those interlayer films which contain no bond strength control agent also whiten as a result of moisture absorption. Our recent research works have revealed that those impurities mentioned below in the resin are involved in the blushing phenomenon as one of the causes thereof.
The interlayer film for laminated glass of the present invention comprises a poly(vinyl acetal) resin as the main component thereof. The process for producing poly(vinyl acetal) resins comprises a step of neutralization. In this neutralization process, an aqueous solution of sodium hydroxide, sodium hydrogen carbonate or the like sodium salt is used. When the sodium salt is used in excess or when another sodium salt is formed as a result of neutralization, the sodium salt may remain in the product poly(vinyl acetal) resin. This residual sodium salt forms particles during polymerization and/or drying, and those particles promote the aggregation of water on the occasion of water absorption by the poly(vinyl acetal) resin, hence serve as a major cause of blushing of the product interlayer film for laminated glass due to moisture absorption. Furthermore, a sodium salt may remain even in poly(vinyl alcohol) in some instances, and this sodium salt may also serve as a cause of blushing of the interlayer film for laminated glass due to moisture absorption in certain instances.
In recent years, the trend toward the use of laminated glass as the automobile side glass screen or in buildings has been increasing and, in these applications, laminated glass is often used with the peripheral portions thereof being exposed. The need for preventing the blushing phenomenon is becoming more and more increased.